Controlled heating method of a process fluid through concentrating solar thermal plant and heat carrier system and apparatus thereof

ABSTRACT

A controlled heating method of a process fluid through concentrating solar thermal plant and heat carrier system where the solar heat available from solar plant is transmitted to a heat carrier stream constituted by air and it is eventually integrated with post firing fuel stream; the heat carrier stream constituted by hot air and post-firing flue gas stream is circulated via a duct circuit through a heat exchanger with the process fluid to allow reaching final required process fluid temperature; the post-firing contribute is determined by an automatic control system which controls in continuous the process fluid temperature and adapt the firing contribute during the different operating phases of the concentrating solar plant.

FIELD OF THE INVENTION

The present invention relates to the field of concentrating solarthermal plants and their application in the traditional refining andpetrochemical sector for heating service purpose. Heating services inrefining and petrochemical sectors, are those normally realized by meansof fired heaters with fuel consumption and combustion products releasedto the atmosphere. Refinery fluids like petroleum, or its fractions orderivatives, or petrochemical process fluids that need to be heated upto a controlled process temperature may be heated up by means of solarheat absorbed and stored through a concentrating solar thermal plant(preferably the type based on parabolic trough concentrators andreceivers and molten salts circulation and storage) with a considerablefuel saving and emission reduction (CO2, SOx, NOx, dusts and unburnedcompounds). Main advantage of solar plants application is to reduce theoverall use of fossil fuels which should be, a part from a mere economicsaving, one target of the sustainable development of industrialcountries, now suffering from a too heavy environmental impact ofindustrial activities and trying to transform their economy toward alarger use of renewable energy. Introducing solar energy application inconventional industrial sectors could be strategic for reaching aneconomy of scale and therefore allowing a wider and faster spread ofthermal solar system.

The use of solar energy in place of fossil fuels has a double benefit onglobal warming, first deriving from direct reduction of fossil fuelsconsumption and related greenhouse gases emissions, second by reductionof the solar energy reflected by earth in the infrared, being this lastone of the main contribute to greenhouse effect.

BACKGROUND OF THE INVENTION

Concentrating thermal solar plant utilizes a “radiation concentratorcollector” which concentrates the solar radiation by focusing it into asmaller area, using mirrored surfaces. In this system, a reflector,which is typically parabolic, receives and reflects (focuses) incomingsolar radiation into a radiation absorber, which is formed as a tube.The tube radiation absorber is conventionally surrounded by a treatedglass enclosure tube to limit the loss of heat and the space between thetube radiation absorber and the glass enclosure tube is under vacuum.

The tube radiation absorber is made of metal with a coating having ahigh solar radiation absorption coefficient to maximize the energytransfer from the solar radiation reflecting off the reflector. Athermal fluid constituting a heat transport medium (either a diathermicoil or molten salts; molten salts mixture is preferred in this case dueto the higher temperature that can be reached by the thermal fluid inthe solar field) flows within the tube radiation absorber and can reachtemperatures up to 580° C. with conventional molten salts mixture basedon Na and K Nitrates.

The thermal energy is then transported by the thermal fluid toward endusers (most commonly steam generators and super-heaters for electricpower generation) and is in part stored in a storage tanks system inorder to extend the operation time. The storage system consists of aminimum of two storage tanks, a cold fluid storage and a hot one. Thesolar heat absorbed and stored in the CSP system can be than carried indifferent ways, have different use, and can be eventually integratedwith a firing contribute to compensate the solar system variationsbefore distribution to selected users.

Main applications of concentrating solar system already realized are inthe field of electric power generation by using hot oil which in turn isused for generating high pressure steam generation and superheatingwhich is then expanded in a steam turbine. More recently the use ofmolten salts was introduced as a way to raise the Rankine cycleefficiency (cit. Patent Application No. EP12167509.4 filed on May 10,2012) and reduce the cost of thermal energy storage (TES).

Recent pilot projects' applications funded by EC include the Matsproject where power, sea water and cold are produced and Comethy projectwhere hydrogen is produced by a low temperature steam reforming processheated by molten salts and integrated with membrane separation (cit.Patent Application No. EP12159998.9 filed on Mar. 16, 2012).

Concentrating solar system have at present a high investment cost butcan significantly reduce the operating cost in those application whereheat required is generated by firing a fuel stream, as it is inconventional heating services of refineries and petrochemicaloperations.

As a background to present invention there are also known applicationsof Heat Recovery Units (HRU) for heating refinery fluids, based on useof Gas

Turbines exhaust streams. In this case the exhaust stream is fullydischarged to stack after heat recovery, at a temperature which dependson the heating service temperature level, usually in the range of 150°C.-300° C., with very large heat dispersion.

With the present invention this is avoided, because a large part of theheat carrier stream downstream heating service is recycled back to theheat carrier circuit, while only a part of the total stream,corresponding to the inlet fuel for post-firing and fresh combustionair, is sent to stack.

SUMMARY OF THE INVENTION

The present invention refers to one specific application ofconcentrating solar thermal plant, based on molten salt circulation andstorage, where the solar heat absorbed by collectors and receivers andstored in the storage system is transferred to an air stream used as aheat carrier, it is eventually integrated/substituted with post-firingunder continuous duty control function, and it is finally conveyed to aheat exchanger where it is transferred to a process fluid. Any heatingservice for refinery fluids, petrochemical process fluids,demineralized/boiler feed water for steam generation and steamsuperheating actually realized through fired heaters is suitable for theapplication of present invention.

One general problem of concentrating thermal solar plant is that ofhaving a variable content of heat available, due to variability of sunradiation along with day time, weather condition, season and geography.If solar heat is to be used for a heating service under temperaturecontrolled conditions, it has to be integrated with a variable heatproducing system that is capable to provide the supplementary heatrequired in a controlled and viable manner.

Object of present invention is the architecture and method of supplyingsolar heat to a refinery or process fluid by means of an intermediateheat carrier circuit having the capacity of integrate the variable heatrequired by process heating service through post-firing. To do this, anair stream, used as a heat carrier between molten salts and final user,is heated up by hot molten salts circulating from solar plant up to atemperature in the range between 480° C. and 550° C., typically 500° C.during the ‘active phase’ (phase 1) when direct sun radiation is heatingmolten salts and during the ‘storage phase’ (phase 2) when the hotmolten salts are taken from the hot storage tank and then discharged inthe cold storage tank. During the ‘inactive phase’ (phase 3) of thesolar field the heat carrier air stream is instead heated up throughpost-firing, realized by means of duct burners, firing a suitable fuel(refinery gas, natural gas, bio-fuel or any mixture of them).

Post-firing is active along all three phases, although only a minimumfiring contribute, with the aim of controlling the process fluidtemperature, shall be used during phase 1 and 2, while during phase 3 itwill provide the full heat duty required by process heating service. Thenecessary post-firing contribute in any phase shall be determined by anautomatic control system the main function of which is to control thefluid service temperature and therefore to adapt the firing contributeduring the different operating phases.

The hot air/flue gas stream is then sent to heat exchange with theprocess fluid and it is recycled back in the circuit to heat exchangewith molten salts and controlled post-firing.

Together with firing contribute, the automatic control system determinesalso the fresh air stream required by fuel combustion and the purgestream, equal in mass to inlet fuel and fresh air stream, to be sent tostack to avoid the build-up of combustion products. During the inactivephase of the solar field the control system will also allow deviation,through an appropriate control valve, of a secondary hot air stream tobe sent to heat exchange with cold molten salt, to eventually avoidmolten salts cooling below a safe temperature (safely above condensationtemperature).

The here described architecture and method for heating a process fluidup to a required temperature is suitable for substituting any fluidheating service up to a temperature of 550° C. or slightly above.

The advantage of the proposed innovation is a meaningful reduction ofthe fuel consumption (in the order of 65%) and greenhouse gas emissioncompared to conventional heating systems.

These and various other characteristics and advantages of the presentinvention will be readly apparent to these skilled in the art uponreading the following detailed description of the preferred embodimentsof the invention, and by referring to the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of the controlled heating system ofa process fluid through concentrating solar thermal plant based onmolten salts circulation and storage according to the present invention.Main Items: 10 Concentrating solar absorption field 11 Hot molten saltsstorage tank 12 Cold molten salts storage tank 20 heat exchanger (moltensalts to heat carrier and vice-versa) 30 post-firing duct burners 40heat exchanger (heat carrier to process fluid) 50 heat carrier circuitfan 60 fresh air inlet fan 70 stack 80 automatic control system 81 inletfluid temperature measure 82 fluid flow-rate measure 83 outlet fluidtemperature measure 84 fuel flow-rate control loop 85 fresh airflow-rate control loop 86 purge stream flow-rate control loop 87 coldmolten salts temperature 88 heat carrier secondary stream valve Fluids:100÷105 molten salts 200 fresh air inlet 201÷210 heat carrier fluid(air/flue gas) 300 fuel to post-firing 400÷401 process fluid to beheated up

DETAILED DESCRIPTION

As illustrated schematically in FIG. 1 main components of the fullarchitecture of present invention consist of a concentrating solarthermal plant 10, a heat exchanger 20 between molten salts and heatcarrier and a heat exchanger 40 between heat carrier and process fluid.

The concentrating solar field 10 and storage system 11 and 12 based onmolten salt circulation and storage is designed for absorbing andstoring the heat necessary for the active and storage phase (about ⅔ ofthe continuous operation time of the required process heating).

The solar collection system 10 comprises one or more tube radiationabsorbers and a plurality of through collectors, such a single axisparabolic reflectors.

Alternatively, any suitable means for concentrating solar radiation,such as Fresnel collectors, may be provided. The thermal fluid is heatedwithin the receiver tubes upon exposure to direct and concentrated solarradiation.

The hot thermal fluid 100 coming from solar field is stored in the hotstorage tank 11 and then circulated by means of suitable pumps (notshown) to provide a heat source to the intermediate heat carrier stream201 through a heat exchanger 20. The heat exchanger 20 transfers heatfrom molten salts to the heat carrier fluid during phase 1 and 2 (activeand storage) while it is suitable also for transferring heat back tomolten salts during phase 3 (inactive solar) to avoid molten saltscooling down below safe temperature (safely above condensationtemperature).

The post-firing 30, realized by means of duct burners located inside theair duct, is capable of firing any capacity from 0% to 100%.

Heat release is calculated in continuous by control system 80 on thebase of heat required by the process fluid 400 to reach its finalcontrolled temperature 83. The firing contribute LHV is delivered as aflue gas stream 300 under flow control 84 in the heat carrier stream 202for the subsequent delivery of heat carrier stream 203 to heat exchanger40 with process fluid 400.

The heat exchanger 40 is capable to transfer heat from the heat carrier203 to the process fluid 400 to reach the final temperature throughtemperature control 83.

The heat carrier circuit fan 50 is capable to provide the pressure headnecessary to the heat carrier fluid necessary to win the circuitpressure drops.

The fresh air fan 60 is capable to admit the required fresh air flow 200under automatic control by means of flow control 85.

The flow control loop 86 is capable to control the purge gas stream tobe sent to stack 70. The control valve 88 permits to derive a heatcarrier by-pass stream to allow temperature control of the cold moltensalts circulating during CSP un-active phase.

1. A controlled heating method of a process fluid through concentratingsolar thermal plant and heat carrier system wherein: the solar heatavailable from solar plant is transmitted to a heat carrier streamconstituted by air; the variable heat supplied by solar thermal plant toheat carrier air stream is integrated with post-firing of a fuel stream;the heat carrier stream constituted by hot air and post-firing flue gasstream is circulated via a duct circuit through a heat exchanger withthe process fluid to allow reaching final required process fluidtemperature; the post-firing contribute being determined by an automaticcontrol system which controls in continuous the process fluidtemperature and adapt the firing contribute during the differentoperating phases of the concentrating solar plant.
 2. A controlledheating method of claim 1 in which a first heat exchanger istransferring heat from molten salts to heat carrier stream during solarplant active phases (direct radiation and storage period).
 3. Acontrolled heating method of claims 1 in which said post-firingcontribute can vary from 0 to full duty requirement.
 4. A controlledheating method as per claim 2 in which a second heat exchanger transfersheat from heat carrier to a process fluid up to a controlled finaltemperature.
 5. A controlled heating method as per claim 1 in whichprocess fluid is either a refinery fluid, a petrochemical process fluid,demineralized water, boiler feed water or steam.
 6. A controlled heatingmethod as per claim 1 in which fresh air flow rate in the duct circuitis determined by automatic control and admitted through relevant controlvalve via a fan.
 7. A controlled heating method as per claim 1 in whicha purge stream to stack is determined by automatic control anddischarged to stack through relevant control valve.
 8. A controlledheating method as per claim 1 in which the first heat exchanger betweenmolten salts and heat carrier stream can work reversely as a heater ofcold molten salt during solar plant inactive phase.
 9. A controlledheating method as per claim 1 in which a heat carrier secondary streamis automatically sent through a control valve to molten salts heatexchanger to avoid cold molten salts solidification during solar plantinactive phase.
 10. A controlled heating apparatus of a process fluidthrough concentrating solar thermal plant and a heat carrier circuit,which comprises: a concentrating solar thermal plant (10) based onmolten salts circulation; a heat exchanger (20) to transfer heat frommolten salts to a heat carrier fluid (201) during solar plant activephases; a post-firing (30) to transfer heat to the heat carrier fluid(202) in output from exchanger (20); a heat exchanger (40) to transferheat from the heat carrier fluid (203) in output from post-firing (30)to the process fluid (400); the post-firing contribute being calculatedby an automatic control system (80) which controls the process fluidtemperature in outlet (83) and adapts the firing contribute during thedifferent operating phases of the solar plant (10).
 11. An apparatusaccording to claim 10, wherein the post-firing (30) is realized by ductburners located inside the air duct.
 12. An apparatus according to claim10, further comprising a heat carrier circuit fan (50) capable toprovide the pressure head necessary to the heat carrier fluid to win thecircuit pressure drops.
 13. An apparatus according to claim 10, furthercomprising a fresh air fan (60) capable to admit the required fresh airflow (200) under automatic control via a flow control (85) into the heatcarrier air-flue gas (201).
 14. An apparatus according to claim 10,further comprising a flow control loop (86) capable to control the purgegas stream to be sent to a stack (70).
 15. An apparatus according toclaim 10, further comprising a control valve (88) which permits toderive a heat carrier by-pass stream (210) to allow temperature controlof the cold molten salts circulating during CSP un-active phases.